Sharks are widely regarded as carnivores (meat-eaters), and until recently the bonnethead shark, Sphyrna tiburo, was no exception. Living in shallow waters close to shore, it was known to hunt in the seagrass meadows for fish, crabs, and shrimp.
However, analysis of the stomach contents of these sharks revealed they were also eating copious amounts of seagrass—especially the juvenile sharks, with up to 62% of their total gut contents identified as plant material.
Unsure whether seagrass-eating was incidental or deliberate, and dubious as to whether the bonnethead sharks could extract any nutritional benefit from it given their too-short (for digesting plant fibrous material) “carnivorous ancestry” intestines, researchers investigated further.1 For three weeks they fed captive bonnethead sharks a daily diet of 90% seagrass (containing a traceable 13C label) and 10% squid. The sharks all gained weight, and more than half of the seagrass was being successfully digested. Furthermore, the researchers observed a spike in the activity of a cellulose-degrading enzyme—possibly belonging to a helpful microbe living in the shark’s gut.
The researchers say these results provide explicit evidence that bonnethead sharks, “animals previously thought to be solely carnivorous”, derive substantial nutritional benefit from eating seagrass.1
Reporting the discovery, Science journal described the bonnethead shark as “the world’s first salad-eating shark”.2 Some might dispute that given longstanding observations in the wild of nurse sharks grazing algae. And also due to the existence of ‘Florence’, a captive nurse shark that since an earlier fishhook injury now shuns meat in favour of broccoli, cabbage, and other greens.3,4
Actually, none of these should be considered the world’s first salad-eating sharks. That honour goes to the first sharks that ever lived, created about 6,000 years ago on Day 5 of Creation Week, and created 100% vegetarian. Today’s salad-eating sharks are a reminder that theirs was originally a herbivorous ancestry, not a carnivorous one, even though in today’s post-Fall world they and other ‘vegetarian carnivores’ might need to augment their diet with some meaty protein from time to time.
References and notes
- Leigh, S., Papastamatiou, Y., and German, D., Omnivorous sharks? An analysis of bonnethead shark digestive physiology provides evidence for seagrass digestion and assimilation, sicb.org, Society for Integrative and Comparative Biology—Annual Meeting 2018, Abstract 109-1, 7 January 2018. Return to text.
- Pennisi, E., Meet the world’s first salad-eating shark, sciencemag.org, 7 January 2018 | doi:10.1126/science.aas9301. Return to text.
- Wrenn, E., Meet Florence, the world’s first vegetarian shark who prefers celery sticks and cucumber to fish, dailymail.co.uk, 18 May 2012. Return to text.
- Catchpoole, D., Vegetarian shark—The nurse shark that uses her razor-sharp, serrated teeth to pulp broccoli and cabbage, celery and lettuce, Creation 36(2):15, 2014. Return to text.